In a treatment method based on a particle beam, there is utilized a high-energy charged particle beam such as a proton beam or a carbon beam accelerated up to 70% of the light velocity. These high-energy charged particle beams have the following features when irradiated into a body. Firstly, almost all of irradiated charged particles stop at a position of the depth proportional to the charged particle energy raised to the 1.7th power. Secondly, the density (referred to as a dose) of energy, which is given to the path through which an irradiated charged particle passes until it stops in a body, becomes maximum at a position where the charged particle stops. A distinctive deep dose distribution curve formed along a path through which a charged particle beam passes is referred to as a Bragg curve. The position where the dose value becomes maximum is referred to as a Bragg peak.
A three-dimensional particle beam irradiation system is contrived in such a way that, while it scans the Bragg peak position in accordance with the three-dimensional shape of a tumor and adjusts the peak dose at each scanning position, a predetermined three-dimensional dose distribution is formed in a tumor region, which is a target preliminarily determined by an imaging diagnosis. The scanning of a charged particle beam stops includes scanning in transverse directions (X and Y directions) that are approximately perpendicular to the irradiation direction of the charged particle beam and scanning in a depth direction (Z direction) that is the irradiation direction of the charged particle beam. In the transverse-direction scanning, there exist a method of moving a patient with respect to a charged particle beam and a method of moving the position of a charged particle beam with an electromagnet or the like; in general, the method utilizing an electromagnet is adopted. Scanning in the depth direction is performed only by changing the energy of a charged particle. As the method of changing energy, there exist a method of changing the energy of a charged particle by means of an accelerator and a method of inserting an energy attenuator into a path through which a charged particle beam passes and changing the attenuation amount of the attenuator.
The method of moving (referred to also as scanning) the position of a charged particle beam with an electromagnet is disclosed, for example, in Patent Document. As illustrated in FIG. 2 of Patent Document 1, in a particle beam irradiation system of a conventional particle beam therapy system, as a means for moving the position of a beam spot, a scanning magnet (scanning electromagnet) that deflects a charged particle beam to the X-Y direction, which is a direction perpendicular to the traveling direction of a beam (the Z direction), is utilized.
A particle beam irradiation apparatus disclosed in Patent Document 2 is configured in such a way that two or more scanning electromagnets which are separately dedicated to the X direction and the Y direction are arranged between a final deflection electromagnet and a deflection electromagnet at the incident side thereof so that a parallel irradiation field is formed through the superimposition of kicks caused by the two or more scanning electromagnets.
With regard to a particle beam irradiation apparatus utilizing a rotating gantry, Patent Document 3 discloses a configuration in which an electromagnet for moving an irradiation field is disposed at the upstream side of a deflection electromagnet and a pair of X-direction and Y-direction scanning electromagnets are arranged at the downstream side of the deflection electromagnet so that the electromagnet for moving an irradiation field largely moves the irradiation field and in the moved region, the scanning electromagnets scan a beam in the X direction and the Y direction.